It has long been a desired object to convert straw left over from the harvesting of wheat, rice, oats, barley, rye, and sugar cane grass-type straws into a source of fuel. Many areas of the world do not have abundant forests and/or minerals capable of producing fuel but have grazing areas in abundance from which straw or grasses may be produced.
The only practical way to use straw as a fuel is to compress it into dense cubes or bricks. Bales are not suitable for efficient fuel use. Loose straw or large bales of straw or grass pose many problems for most fuel systems, such as handling and introduction into the combustor. Most solid fuel combustors are designed to handle coal or chunk type fuels that can be conveyed, stored, and metered using augers or chain conveyors. Holding areas (solos and bunkers), preparation machinery (grinders and pulverizers), and metering systems cannot handle loose straw, grass or even bales (approx. 3'.times.2'.times.1.5' to 4'.times.4'.times.8'). Pulverization equipment such as grinder-blowers or ball mills for solid fuel injection systems, are all designed to handle heavy, solid chunk type fuels such as coal.
Actual combustion is also a problem for loose fuel. Solid fuel combustors traditionally have zones designed for maximum efficiency which are usually 2 to 8 feet above the bed of the combustor. Loose or light fuels entering the combustor fly up into the vapor space, burning out of the zone for maximum efficiency and causing problems with the economizers and multi-clone systems. Chunk or cube type fuels with a unit density of up to 60 pounds per cubic foot drop into and stay in the bed area releasing heat in the zone designed for maximum efficiency.
Attempts to make cubes from straw or grass, have, however thus far met with failure. Straw and grass has a crystalline lattice structure in the cellulose and will not stay compacted even after being subjected to intense pressure. In the past, attempts have been made to make straw or grass cubes by adding various chemicals to the straw prior to feeding the straw or grass into a cuber or densification machine.
For example, calcium ligno-sulfonate, bentonite, dolomite, and calcium hydroxide have all been added to straw in an attempt to aid in the formation of dense cubes which could be then used as a fuel. Each additive has drawbacks. For example, calcium ligno-sulfonate melts and becomes too gummy to form a good cube and forms a residue that causes the cubers or densification machines to be hard to start after cooling. Cubes made with bentonite do not hold together well unless a very high percentage is used. This is impractical, first because of the cost involved, and also because these high percentages of bentonite are very abrasive and wear out the dies and internal parts of the cubers very rapidly. Also, bentonite and bentonite-type binders are based on diatomaceous earth and their use in high percentages results in a fuel with a very high ash content. Calcium hydroxide will not produce a good quality cube unless used in percentages usually exceeding 10% to 15%. The costs associated with this chemical become prohibitive at these use levels.
Thus, to date all efforts to make cubes from straw and grasses have been unsuccessful because the material added to the straw to make it bind together either makes unsatisfactory cubes or prevents the densification machine from operating correctly.
Densification machines of this type are more commonly known as cubers or pelletizers. These devices make compacted cubes of material by forcing loose fluffy starter material through an array comprising a ring of dies under intense pressure. Material is usually fed into a mixer from which it is forced into the path of a press wheel which pushes the material through a die array. Conventional cubers of this type are manufactured by Warren & Baerg Company of Dinuba, California. Pellet machines are manufactured by various companies including California Pellet Mills.